Habitat - The location where a plant or animal lives. This location supplies the organism with the various abiotic and biotic conditions it requires for survival. Many organisms occupy only a fraction of the possible habitat sites. We refer to this as their realized niche. 

Habitat Fragmentation - A process that causes the breakup of natural habitat into smaller pieces that are then interspersed by fragments of developed land used for human purposes, like forestry, agriculture, grazing, or settlement. This process causes the restricted movement of native species, as a human-developed fragment often acts as a barrier to dispersal and migration. Restricted dispersal and migration can isolate populations and cause genetic diversity to decline over time.

Habitat Islands - Are fragments of natural habitat surrounded by human-developed pieces of land that are used for forestry, agriculture, grazing, or settlement. They arise from habitat fragmentation.

Haboob - An intense dust storm caused by strong wind. Common in arid regions of the world, these storms are often generated by the downdrafts that occur at the end of the life of a thunderstorm. 

Hadal Zone - The deepest vertical zone within an ocean, usually more than 6,000 meters (19,600 feet). Also see (in order of depth in a water body) pelagic zone, photic zone, aphotic zone, mesopelagic zone, bathyal zone, abyssal zone, demersal zone, and benthic zone.

Hadean - Geologic eon that occurred from 3,800 to 4,600 million years ago. The Earth's oldest rocks formed at the end of this period. Also see Archean, Proterozoic, and Phanerozoic Eons. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Hadley Cell - Three-dimensional atmospheric circulation cell located at roughly 0 to 30° North and South of the equator. The Hadley Cell consists of rising air (Intertropical Convergence Zone) at the equator and descending air (Subtropical Highs) at 30° North and South. At Earth's surface, this global-scale circulation system generates the Northeast and Southeast Trade Winds. Compare with Ferrel Cell and Polar Cell.

Haff - An isolated coastal lagoon that has become cut off from the open ocean by a spit.

Hail - Hail is a solid form of precipitation that has a diameter greater than 5 millimeters. Occasionally, hailstones can be the size of golf balls or larger. Hailstones of this size can be quite destructive. The presence of intense updrafts in mature thunderstorm clouds is a necessary requirement for the formation of hail.

Hair Hygrometer - A meteorological instrument that uses the expansion and contraction of hair to determine atmospheric humidity.

Half-Life - The time required for one-half of the nuclei in a radioisotope to emit its radiation. The half-life for radioisotopes ranges from a few millionths of a second to several billion years.

Halite - A sedimentary rock created by the chemical precipitation of sodium and chlorine. The chemical formula for halite is NaCl.

Halo - An optical phenomenon where a bright ring forms around the moon or the Sun. Halos form when light is refracted by ice crystals in the atmosphere. The ice crystals responsible for halo development are part of cirrus and cirrostratus clouds located about 5 to 10 kilometers (3 to 6 miles) above the ground surface. Also called a 22° halo. 

Haloclasty - The weathering and disintegration of rocks and minerals by the growth and expansion of salt crystals. An important weathering process in arid climates. 

Halokinesis - The process of subsurface salt movement in geologic structures. Halokinesis causes the accumulation of large amounts of salt in salt domes.

Halomorphic Soil - Is a soil that has experienced an accumulation of salt at or near its surface.

Halons - Ethane or methane-based compounds that have had some of their hydrogen ions chemically substituted by chlorine, fluorine, and/or bromine. One member of this group of chemicals is chlorofluorocarbons (CFCs). Halons are relatively strong greenhouse gases. These chemicals also contribute to the depletion of stratospheric ozone. The Montreal Protocol requires industrialized nations to phase out the manufacture and use of halons.

Halophyte - A type of plant that has the ability to successfully grow in environments with high concentrations of sodium chloride (salt).

Hamada - A flat desert area of mainly exposed bedrock.

Hanging Glacier - An alpine glacier where its end hangs over a steep slope and continually breaks off, causing ice avalanches. 

Hanging Valley - A secondary valley that enters a main valley at an elevation well above its floor. Hanging valleys are the result of past erosion caused by alpine glaciers. Hanging valleys are often the site of waterfalls.

Hanging Wall - The topmost surface of an inclined fault.

Haploid - A type of cell that contains only one set of chromosomes. Also see diploid.

Hardness - (1) A measure of a mineral's ability to scratch on the ten-point Mohs hardness scale. On this scale, the assigned values are: 1 - talc; 2 - gypsum; 3 - calcite; 4 - fluorite; 5 - apatite; 6 - feldspar; 7 - quartz; 8 - topaz; 9 - corundum; and 10 - diamond. This scale is used for mineral identification. (2) A measure of the concentration of dissolved solid cation minerals in a sample of water. The most common cations found in water are calcium (Ca₂⁺) and magnesium (Mg₂⁺). Several different units are used to estimate hardness, including conductivity and total dissolved solids.

Hardpan - An impervious layer found within the soil. Results from the precipitation of iron, illuviation of clay, or the cementing of sand and gravel by calcium carbonate precipitates. A hardpan composed mainly of calcium carbonate is known as a calcrete. See iron pan.

Harvesting - Is the human capture of a large number of individuals from a particular plant or animal species for the purpose of food production or the making of commercial goods.

Hawaiian Eruption - A type of volcanic eruption typified by lava that originates from basaltic magma with less than 1 percent dissolved water. The low dissolved water content prevents the eruption from being explosive. Instead, this eruption type is characterized by gentle surface flows of very fluid lava. Lava generated by Hawaiian eruptions also has a low gas content, high vent temperature, and produces only small amounts of volcanic ash. 

Hawaiian High - See Pacific High.

Hazard - Something that can cause loss of life, injury, disease, economic loss, or environmental damage. Hazards can be classified as natural or human-caused.

Haze - A condition of reduced atmospheric visibility caused by the presence of suspended particles consisting of aerosols, water, and/or smog. The particles involved are usually less than one micrometer (µm) in diameter. Reduced visibility is caused by suspended particles scattering incoming sunlight. The most common types of solid particles involved in haze include dust, smoke, and volcanic ash.

Headcut - The upslope starting point for a gully. The process of gully erosion causes the headcut to migrate upslope over time.

Headland - A strip of land that juts seaward from the coastline. Headlands are normally bordered by a coastal cliff.

Headward Erosion - Is the situation where a stream erodes the sediment and rock at its headwaters in the opposite direction that it flows. This causes the stream's long profile to become longer.

Headwaters - The upper portion of a stream's drainage system.

Heat - Heat is defined as energy in the process of being transferred from one object to another because of the temperature difference between them. In the atmosphere, heat is commonly transferred by conduction, convection, advection, and radiation.

Heat Balance - A concept that suggests incoming radiation received by an object is in balance with outgoing radiation. For example, the solar radiation received by the Earth is balanced by its outgoing longwave radiation emissions.

Heat Budget - An estimate of how various types of energy and heat energy are stored and used in some system of interest. A system's heat budget can be described using an equation or a graphical model.

Heat Capacity - Is the ratio of the amount of heat energy absorbed by a substance compared to its corresponding temperature rise.

Heat Energy - A form of energy created by the combined internal motion of atoms in a substance.

Heat Equator - See thermal equator.

Heat Island - A dome of relatively warm air that develops over the center of an urbanized area. Caused mainly by the unique solar radiation absorption and heat energy storage characteristics of the built environment and by the artificial generation of heat by urban activities like the artificial heating of buildings, industrial processes, and vehicle traffic.

Heat Wave - A long period of weather where temperatures are well above average. The discomfort associated with a heat wave can be amplified by high humidity. Many different definitions exist to identify heat waves.

Heating Degree Day - For a single day, this climatological measurement determines the number of degrees that the daily mean temperature is below a specified base temperature. For example, if the daily mean temperature was recorded as 10°C and the base temperature was  24°C, the heating degree day measurement would be 14°C.  Sometimes these daily departures are determined for a period longer than one day. In this case, the departures are summed together to get a cumulative total of heating degree days for that period. Compare with a cooling degree day.  

Heavy Metal - Is a metal that has a relatively high density, atomic number, and/or atomic weight. Heavy metals are relatively uncommon in Earth's rocks, sediments, and soils, but are often used in the things humans construct. Some heavy metals (such as arsenic, cadmium, copper, lead, nickel, and silver) are highly toxic to organisms at relatively high concentrations.

Heavy Soil - A soil that has a high proportion of fine particles, like clay.

Hectare (ha) - A metric unit of area, two-dimensional measurement, with no predetermined shape, used in most of the world. One hectare is equivalent to 10,000 square meters or 0.01 square kilometers. Converting one hectare to imperial units equals 107,639 square feet or 2.471 acres.

Helical Flow - Movement of water within a stream that occurs as spiral motions.

Helictite - A type of speleothem that has a non-vertical long axis. One theory for their formation is that wind plays a role.

Heliophyte - A plant adapted to survive best in full intensity sunlight.

Hematite - Mineral with the chemical composition iron oxide (Fe₂O₃). Hematite ore is mined to produce the metal iron.

Hemera - A period in the geological history of the Earth that is identified by the presence of particular fossils.

Hemipelagic Deposits - Sediment deposits found on the continental shelf and continental slope.

Herb - A non-woody angiosperm whose above-ground vegetation dies off seasonally.

Herbicide - A chemical substance that causes reduced growth and/or reproduction or the death of plants. Some herbicides have been developed to selectively kill particular types of plants.

Herbivore - A heterotrophic organism that consumes plants for nutrition. Also known as a primary consumer. Compare with detritivore, omnivore, scavenger, and carnivore.

Heredity - The metabolic transmission of behavioral, physiological, and morphological characteristics from parent to offspring. These characteristics are known to be mainly passed down to subsequent generations through DNA. Recent research has discovered epigenetic inheritance systems. 

Heterogeneity - The state of being different or diverse.

Heterosphere - The upper layer in the atmosphere that is identified by the fact that the gases nitrogen, oxygen, helium, and hydrogen dominate at specific altitudes because of their molecular weight. Zones of gradual transition separate the gas's distinct layers in the heterosphere. The heterosphere extends upward from a height of 80 to 100 kilometers (50 to 62 miles), depending on latitude. Below the heterosphere is the homosphere.

Heterotroph - An organism that must consume energy-rich organic molecules for survival. Energy is released from these molecules through the chemical process of cellular respiration. Also see carnivores, herbivores, and detritivores.

Hiatus - A break in the recording of geologic time as chronicled by successive layers of strata in a stratigraphic deposit. 

High Pressure - An area of atmospheric pressure within the Earth's atmosphere that is above average. If this system is on the Earth's surface, has circular wind flow, and enclosed isobars, it is called an anticyclone.

High Tide -  The highest elevation of the ocean surface during a 24-hour and 50.4-minute tidal cycle. Same as high water mark. Compare with a low tide.

High Water - This term refers to the maximum elevation of the ocean surface during a 24-hour and 50.4-minute tidal cycle for diurnal or semi-diurnal tides. Same as high tide. Compare with the low water.

High Water Mark - (1) The elevation of the ocean surface along a shoreline at high tide. Compare with the low water mark. (2) A documented elevation of some past extreme height of a body of water on the adjacent land surface. These high water marks often occur because of flooding. 

Higher High Water - Term used to describe the higher event of the two maximum ocean surface elevations in a mixed tide during a 24-hour and 50.4-minute tidal cycle.

Higher Low Water - Term used to describe the higher event of the two minimum ocean surface elevations in a mixed tide during a 24-hour and 50.4-minute tidal cycle.

Histogram - A type of graph produced exclusively to display the shape of the data associated with a single variable. The X-axis shows the range of values found in the dataset. The y-axis displays the frequency count. Bars are used to show the number (frequency) of observations found in equal class intervals. Histograms are commonly used to determine if the data has a normal distribution. 

Historical Geology - The sub-discipline of Geology that studies the geologic and evolutionary history of Earth.

Histosols - Soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. This soil is composed of more than 30% organic matter as a result of saturated environmental conditions.

Hjulström Curve - A graph that describes the relationship between stream flow velocity and particle erosion, transport, and deposition. First described by Swedish geomorphologist Filip Hjulström in 1935.

Hoar Frost - A type of frost that develops on the surfaces of vegetation and other objects as delicate white crystals of ice. Hoar frost usually forms on cold, clear evenings, when objects become colder than the surrounding air because of longwave radiative heat loss.

Hodograph - A graph used to plot the velocity (in knots, miles per hour, kilometers per hour, meters per second, etc.) and direction (usually out of 360°) of a body or a fluid at different heights above a surface for a given moment. Commonly used in meteorology to plot the change in wind speed and direction with height above the ground. A hodograph is very useful to meteorologists for estimating vertical wind shear, atmospheric turbulence, and temperature advection. Data for constructing a hodograph typically come from a vertical sounding in the atmosphere, obtained from a radiosonde. Also called a velocity diagram. 

Hogback - Is an elongated, narrow ridge or series of ridges with a slender peak and steeply sloping sides. These geomorphic features can vary significantly in size. Hogbacks often form from the differential weathering and erosion of steeply dipping sedimentary strata. Usually, the ridge forms because it is made of rock that is more resistant to weathering and erosion. 

Holarctic - The zoogeographical region or biogeographic realm that is made up of both the Nearctic (North America) and Paleoarctic (Europe and Asia) regions. These regions have similar faunas because, in the past, they were connected by the Bering land bridge.

Holistic - Being concerned with the complete system level of understanding of a phenomenon. Holism is the opposite of a reductionist approach to studying and understanding phenomena.

Holocene - The geologic epoch from about 10,000 years ago to today. During this period, most glaciers retreated because of a warmer global climate. The Holocene is also the time of modern humans. One of two epochs during the Quaternary Period. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Holocene Megathermal - See Climatic Optimum.

Holocene Optimum - See Climatic Optimum.

Holocene Thermal Maximum - See Climatic Optimum.

Homoclime - Places on our planet that generally have similar climates as determined by monthly and annual measurements of temperature and precipitation.

Homocline - Layers of rock strata that dip uniformly in one general direction. A homocline can represent one limb of an anticline or syncline.

Homeostasis - A situation where a biotic or abiotic system has the ability to maintain equilibria in the various processes operating within its boundary. This is done through negative and positive feedback processes that lead to a change in the system's state. 

Homeostatic (Homeostasis) - A constant or non-changing state of equilibrium in a system despite changes in external conditions.

Homeostatic System - A system that maintains homeostasis.

Homoiothermy - The ability of some organisms to maintain an internal body temperature that is relatively constant and independent of the temperature in their surrounding environment. One group of organisms that has this ability is mammals. These organisms are often called warm-blooded.

Homosphere - The lower layer in the atmosphere that is identified by the general homogeneity of chemical composition. In this layer, nitrogen, oxygen, argon, carbon dioxide, and trace gases dominate and maintain constant relative proportions throughout the layer. The homosphere extends from the Earth's surface to a height of 80 to 100 kilometers (50 to 62 miles), depending on latitude. Above this atmospheric layer is the heterosphere.

Hoodoo - Capped columns of rock extending upward from the ground surface about 1.5 to 45.0 meters (5 to 150 feet) tall. These geomorphic features are found in arid landscapes and form when a stratum of more resistant rock caps an extensive layer of softer rock. Hoodoos form because the cap of more resistant rock protects the softer rock below it from the effects of weathering and erosion.

Horizon - (1) A surface separating two beds in sedimentary rock. (2) A layer within a soil showing unique pedogenic characteristics. Four major horizons are normally found in a soil profile: A, B, C, and O. (3) The point at which the visible edge of the Earth's surface meets the sky.

Horn - A pyramidal-shaped peak that forms when several cirques erode a mountain from three or more sides.

Hornblende - A common mineral belonging to the amphibole group that consists of silicates of calcium, iron, and magnesium. Found in many igneous and metamorphic rocks.

Horse Latitudes - Refers to the two areas north and south of the equator located between 30 and 35° latitude. Within this area, the Subtropical High Pressure Zone usually dominates, creating consistently dry and warm weather over water and dry and hot weather over land. Winds are generally light in this area, and cloud cover and precipitation are not common.

Horst Fault - A fault that is produced when two reverse faults cause a block of land to be pushed up.

Horton Overland Flow Model - A simple mathematical model that estimates the quantity of overland flow per unit area. R.E. Horton first proposed this model in 1933. This model suggests that overland flow equals rainfall minus the soil's infiltration capacity. In this model, infiltration capacity is estimated with the following equation:

f = fc +(fo - fc) exp(-ct)

where f is the infiltration capacity, fc , fo, and c are constants that vary with the type of soil and the distribution of water held within the soil, and t is time.

Hortonian Analysis - A morphometric technique that calculates the relationship between variables such as the number of stream segments, stream channel lengths,  and drainage basin areas for particular stream orders. This technique involves determining the bifurcation ratio. This value is calculated as the ratio of a calculated value for one order to the next lower order. See Law of Stream Number, Law of Stream Lengths, and Law of Basin Areas.

Host - An organism that develops a disease from a pathogen or is being consumed by a parasite.

Hot Spot - A volcanic area on the surface of the Earth created by a rising magma plume in the mantle.

Hot Spring - A continuous ground flow of water originating below the Earth's surface and having a temperature higher than 37° Celsius (98.6° Fahrenheit). 

Hum - An isolated hill that survived the lowering of a landscape dominated by limestone bedrock by the processes of weathering and erosion.

Human Geography - A field of knowledge that studies human-made features and phenomena on the Earth from a spatial perspective. Sub-discipline of Geography.

Human-Land Tradition - An academic focus in modern Geography that investigates human interactions with the environment.

Humic Acid - A collection of acidic substances derived from the decomposition of organic substances. These substances are commonly found in soil, peat, coal, and mixed in the water of streams, lakes, and the ocean. Humic acids often form chemical complexes with cations, creating humic colloids.

Humic Layer - A common near-surface layer in well-developed soil that consists of highly decomposed organic matter. Compare with fibric and mesic layers.

Humid - An environment or habitat or location that has a climate similar to a moist tropical rainforest or temperate rainforest. Moisture, humidity, and precipitation are usually plentiful year-round. Compare with subhumid.

Humidity - A general term used to describe the amount of water vapor found in the atmosphere. Also see relative humidity, vapor pressure, and mixing ratio.

Humidex - This calculation combines the effects of temperature and humidity to measure the degree of discomfort an individual would experience in warm weather. This measure was developed by Canadian meteorologists in 1965 and is commonly used in Canada. The equation used to determine humidex is:

Humidex = T + (0.5555 x (e - 10))

where T is the temperature in °C and e is the vapor pressure in millibars (mb). At values greater than 40, most unnecessary outdoor activities should be reduced. If values are between 35 and 39, strenuous outdoor exercise should be reduced. Humidex measurements between 20 and 29 are considered comfortable. In the United States, this type of measure involves a different calculation and is called the discomfort index. 

Humification - The chemical process where mineralization recombines soluble acids in the soil, causing the formation of humus.

Humus - A dark-colored, semi-soluble organic substance formed from the decomposition of soil organic matter.

Hurricane - A powerful cyclonic storm consisting of a mass of thunderstorms organized around an intense low pressure center. Hurricanes only develop over the warm oceans of the tropics. To be classified as a hurricane, wind speeds in the storm must be greater than 118 kilometers per hour (73 miles per hour).

Hydration - A form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

Hydraulic Force - The force created by a mass of water flowing under the influence of gravity and/or by the change in the shape of the solid boundary that confines this water flow.

Hydraulic Fracturing - See hydrofracturing.

Hydraulic Friction - The frictional resistance imparted to flowing water in a stream by the roughness of the stream bed.

Hydraulic Gradient - The slope of the water table or aquifer. The hydraulic gradient influences the direction and rate of groundwater flow.

Hydraulic Radius (R) - A measurement associated with streams that is related to stream flow velocity.  Velocity of flow increases with an increase in hydraulic radius. Hydraulic radius is determined by taking the channel cross-sectional area (A) and dividing it by the wetted perimeter (P) [R = A/P].

Hydraulics - The scientific study of the flow of liquids.

Hydric Layer - The layer of water on organic soils that starts at the ground surface and extends to a depth of between 40 and 160 centimeters (16 to 63 inches).

Hydrocarbon - A common organic compound composed primarily of hydrogen and carbon atoms. Examples of hydrocarbons include coal (various mixtures of the elements carbon and hydrogen with sulfur, oxygen, and nitrogen), petroleum (C_nH_2n+2), oil, and methane (CH₄).

Hydrofracturing - A process where water under pressure enters naturally occurring small cracks in rocks, causing subsequent rock fracturing. This process can be enhanced by the presence of salts or by temperatures falling below 0°C, when freezing of water causes volumetric expansion. This technique is being used to increase the recovery of natural gas and petroleum from wells for human use. When used to extract natural gas and petroleum, water is usually mixed with silica sand and between 3 and 12 additive chemicals. Some of the additive chemicals include hydrochloric acid, acetic acid, sodium chloride, polyacrylamide, ethylene glycol, borate salts, sodium and potassium carbonates, glutaraldehyde, guar gum, citric acid, and isopropanol. Also called hydraulic fracturing, fracking, and fraccing.

Hydrogenic Soil - A soil that forms under an environment of continuous waterlogging.

Hydrogeology - The scientific study of the storage and movement of groundwater.

Hydrogeomorphology - The geomorphological study of water as an agent in landscape processes and forms.

Hydrograph - Is a graph describing the change in stream discharge over time produced by a recording instrument located somewhere along the course of a stream. 

Hydrologic Cycle - A generalized model that describes the movement of water between the hydrosphere, lithosphere, atmosphere, and biosphere (see graphic below). Also called the hydrological cycle.  

Hydrological Cycle - See hydrologic cycle.

Hydrology - A field of Physical Geography and Earth Science that studies the hydrosphere.

Hydrolysis - A chemical weathering process that involves the reaction between mineral ions and the ions of water (OH^- and H⁺), and results in the decomposition of the rock surface by forming new compounds, and by increasing the pH of the solution involved through the release of the hydroxide ions.

Hydrometeorology - An academic discipline that applies the knowledge associated with meteorology to problems and phenomena common to hydrology. 

Hydrometer - A scientific instrument that measures the specific gravity of liquids.

Hydrometry - A term that describes the measurement and monitoring of water-associated phenomena within some component of the hydrologic cycle. Hydrometry includes the measurement and monitoring of stream flow, groundwater flow, precipitation, and water quality.  

Hydrophyte - A plant that has the ability to successfully grow in only water or very saturated soils. Hydrophytes that grow best in water are also known as aquatic plants.

Hydrosphere - The hydrosphere describes the movement and storage of water on Earth. Water exists on the Earth in various stores, including the atmosphere, oceans, lakes, streams, glaciers, snowfields, and groundwater. Water moves from one store to another through evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, and groundwater flow.

Hydrostatic Pressure - The force exerted by water when it is under pressure.

Hydrothermal Vent - A volcanic fissure that releases geothermally heated water. Earth has many hydrothermal vents in the oceans at the mid-oceanic ridges.

Hydroxide - (1) An anion made up of an atom of oxygen and hydrogen. Has the chemical formula of OH^-. (2) A compound that consists of the joining of some chemical element to the radical OH^-.

Hygrograph - A mechanical device used to create a record of changes in atmospheric humidity for a location. This record is often made on a paper graph. 

Hygrometer - Any instrument used for measuring atmospheric humidity.

Hygroscopic - A substance that has the ability to absorb atmospheric water and therefore accelerate the condensation process.

Hygroscopic Coefficient - The maximum limit of hygroscopic water around the surface of a soil particle.

Hygroscopic Nuclei - Minute particles of dust, salt, sulfur dioxide, and smoke that attract water vapor to their surface, where it condenses or deposits even when the atmosphere is not saturated. Also see condensation nuclei and deposition nuclei.

Hygroscopic Water - Water held within 0.0002 millimeters (0.0000079 inches) from the surface of a soil particle. This water is essentially non-mobile and can only be removed from the particle through heating.

Hypabyssal Rock - A type of intrusive igneous rock that forms at shallow to medium depths in the crust. Hypabyssal rocks often form dykes, sills, laccoliths, lopoliths, or phacoliths. These rocks are fine to medium-grained. Three examples of hypabyssal rocks include diabase, lamproites, and kimberlites. Also called subvolcanic rock. 

Hypha (Hyphae plural) - Branching thread like structure found on a fungus. Hyphae consist of one or more cells, and in many fungi, this structure represents the main way the organism grows.

Hypogene - (1) Refers to processes beneath the Earth's surface that normally cause the formation of primary minerals. (2) Geologic processes that occur at very deep depths below the Earth's surface.

Hypolimnion - A layer in a lake stratified according to water temperature found below the topmost epilimnion layer and the middle transitional thermocline layer. The water in this layer typically has a lower temperature, pH, and dissolved oxygen concentration than the epilimnion layer above it.

Hypothermal - (1) In Geology, this term refers to a mineral deposit that formed in an environment of relatively high temperature and pressure. (2) A dangerously low body temperature.

Hypothesis - A tentative assumption that is made for empirical scientific testing. A hypothesis becomes a theory when repeated testing and evidence suggest it is likely to be correct.

Hypothesis Testing - A logical process where an alternative and a null hypothesis are statistically tested to falsify a hypothesis. 

Hypoxia - Something that is being deprived of oxygen.

Hypsithermal - The warmest period during the Holocene Epoch. This period spans about 7,000 to 3,000 BCE. During this time, average global temperatures were 1 to 3°C (2 to 5°F) warmer than they are today. Also called Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, and Holocene Thermal Maximum.

Hypsometry - The science of measuring the elevation of the ground surface or solid surface in lakes and oceans. Measurements are usually made relative to a base datum, such as mean sea level.